The present invention relates to amplifiers, and more particularly a multiple input amplifier having an input selection feature.
It is sometimes necessary in the electronic circuitry art to have a multiplicity of inputs for obtaining a singular output, wherein the input to be selected is determined by selection signals.
Amplifiers are typically known in the electronic circuits art to be devices with a single input and a single output, wherein the input is amplified by an amplification factor and provided at the output.
In order to selectively amplify a multiplicity of input signals, either a multiplicity of amplifiers for each input signal is required or the input signals need to be selected utilizing a multiplexor prior to amplification by a singular amplifier. Typically in the prior art, the second approach is utilized and a multiplexor and amplifier are connected in stages. The multiplexor receives a plurality of input signals and a plurality of select control signals. The multiplexor provides unitary gain to the input signals and selects which input signal is to be amplified based on the select signals. The output of the multiplexor is therefore connected to the input of the amplifier. A multiplicity of signals are thus selectively amplified in this manner.
Typically, multiplexors in the prior art have been either relatively slow acting devices or semiconductor devices with a low degree of isolation between inputs, so that abnormal excursions of one input line typically effects other input lines. Another disadvantage to using multiplexors in combination with amplifiers is the fact that the two functions typically require different semiconductor processes and are thus difficult to integrate (i.e. multiplexors require CMOS technology and amplifiers are best utilizing bipolar technology).
Efforts to improve such systems have lead to continuing developments to improve their versatility, efficiency, practicality, speed and costs.
An object of the present invention is to provide an amplifier circuit that provides a multiplicity of input terminals configured such that a single output is generated.
Another object of the present invention is to provide an amplifier circuit that provides a multiplicity of input terminals configured such that when one or more of the input signals is activated the result is an output signal proportional to the input.
Another object of the present invention is to provide an amplifier circuit that provides a multiplicity of input terminals such that when more than one input is activated the output represents the sum of the activated inputs.
According to the present invention, a multiplexing amplifier circuit which receives a plurality of input signals selected by a plurality of select control signals includes
a differential amplifier which provides an output signal indicative of the input signal across a positive input and a negative input;
a selection circuit corresponding to each input signal having a longtail pair of first and second transistors wherein the first transistor receives the input signal and drives the negative input of the differential amplifier and the second transistor receives the feed back output signal and drives the positive input of the differential amplifier;
a pair of diodes connected to each transistor for steering the current through the transistor;
wherein each select signal biases the corresponding diodes to thereby cause its corresponding longtail transistor pair to provide the corresponding input signal across the differential amplifier inputs.
The present invention provides for a multiplexing amplifier circuit having a high degree of isolation of inputs from each other such that only the activated input contributes to the output. In other words there is a low level of cross talk between the inputs.
The present invention also provides the ability to block very high abnormal voltages on inactivated inputs thereby keeping them from contaminating the output.
The present invention can be implemented in a number of semiconductor technologies, including bipolar and junction FET.
The present invention lends itself particularly well to monolithic integration with bipolar semiconductor technology, thereby providing a very low cost, very small composite device.
The present invention provides a selector or multiplexor with very high speed.
The present invention can be implemented making use of existing operational amplifier technology thereby reducing the risk of monolithic integration development.
The present invention is well suited for automated assembly utilizing surface mount components when implementation using discrete components is elected.
The present invention also provides the ability to accommodate completely floating inputs (isolated from circuit ground).
To the accomplishment of the foregoing in related as, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annex drawing set forth in detail certain illustrated embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principals of the invention may be employed. Other objects, advantages, and new features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.